The invention presented herein deals in the art of apparatus for testing and determining the validity of a paper being offered as a valid security, note, currency, or the like. In the past, security validation devices have been developed wherein grid patterns on the security are tested by means of a reticle and photocell positioned on a side of the paper opposite that of a light source. Movement of the reticle and/or the paper creates relative movement between the reticle and grid pattern and a determination of the validity of the paper is then made upon the basis of the registrations achieved between the reticle and the grid pattern. However, with the advent of modern-day photocopying devices capable of reproducing documents to the finest detail, it has been found that prior art validation devices may accept photocopies as valid securities due to the high quality of reproduction of the test grid by the photocopy device.
It has thus become necessary to extend the validation test currently utilized to incorporate tests upon other characteristics of the security which are present in a valid security but absent in a photocopy. To this end, it has been found that the optical density of a note and photocopy are generally quite different and that a test operating upon both a characteristic grid pattern and the optical density of the paper will not easily be deceived by a photocopy or other facsimile.